DNA Molecular Weight and Copy Number Calculator
Calculate DNA molecular weight, molar amount, concentration, and estimated copy number from DNA length and mass. Useful for plasmid preparation, cloning workflows, qPCR standards, library preparation, and DNA normalization experiments.
Calculator
Estimate molecular weight, molar amount, and copy number from DNA length and mass.
Results
Bench Summary
Lab Summary Card
Formula and Calculation Logic
DNA length determines molecular weight because each base pair or nucleotide contributes mass. Double-stranded DNA uses an average molecular weight of approximately 660 g/mol per base pair, while single-stranded DNA uses approximately 330 g/mol per nucleotide.
For concentration inputs, mass concentration is converted to g/µL, divided by molecular weight, and multiplied by 1,000,000 to obtain mol/L. Molecular weight is usually an intermediate step: molar amount, molar concentration, and copy number are the values used to compare DNA molecules experimentally.
Example Workflow
For a 5 kb double-stranded plasmid with 100 ng of DNA:
Molecular weight = 5,000 bp × 660 g/mol = 3.3 × 106 g/mol.
Molar amount = 3.03 × 10-14 mol = 0.0303 pmol = 30.3 fmol.
Estimated copy number = 1.82 × 1010 plasmid molecules.
Common Mistakes
A 5 kb plasmid is 5,000 bp. A unit error changes every downstream result by 1,000-fold.
One microgram is 1,000 nanograms. Confirm the mass unit before comparing samples.
Equal masses of differently sized plasmids contain different numbers of molecules.
Molecule count does not account for DNA quality, topology, delivery efficiency, expression, or infectivity.
Frequently Asked Questions
What is DNA molecular weight?
DNA molecular weight is the estimated mass of one mole of a DNA molecule. It is commonly calculated from sequence length using an average mass per base pair or nucleotide.
Why does plasmid size affect copy number?
Larger plasmids have a greater molecular weight, so the same DNA mass contains fewer plasmid molecules.
What is the difference between copy number and concentration?
Copy number is the estimated number of molecules in a sample. Concentration describes an amount per volume, such as ng/µL, mol/L, or copies/µL.
Why do cloning and transfection protocols use molar amounts?
Molar amounts compare numbers of molecules directly, which is useful when DNA fragments or plasmids have different lengths.
Is 660 g/mol per bp exact?
No. It is a practical average. Exact molecular weight depends on sequence composition, strand ends, modifications, counterions, and hydration state.